[PATCH v3] mm,writeback: fix divide by zero in pos_ratio_polynom

[Rik van Riel <riel@redhat.com>]

On Wed, 30 Apr 2014 15:48:26 +0200
Michal Hocko <mhocko@suse.cz> wrote:

> This is still prone to u64 -> s32 issue, isn't it?
> What was the original problem anyway? Was it really setpoint > limit or
> rather the overflow?

This patch should avoid math overflows with both the initial
subtraction, and with use of the truncated divisor by div_s64
and div_u64.

I added redundant casts in the div_s64 and div_u64 calls to
make it clear what those functions do internally, which should
make it easy to understand why we do the same cast in the if
statements right above.

I believe this version of the patch addresses everybody's concerns.

---8<---

Subject: mm,writeback: fix divide by zero in pos_ratio_polynom

It is possible for "limit - setpoint + 1" to equal zero, leading to a
divide by zero error. Blindly adding 1 to "limit - setpoint" is not
working, so we need to actually test the divisor before calling div64.

Signed-off-by: Rik van Riel <riel@redhat.com>
---
 mm/page-writeback.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index ef41349..6405687 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -598,10 +598,15 @@ static inline long long pos_ratio_polynom(unsigned long setpoint,
 					  unsigned long limit)
 {
 	long long pos_ratio;
+	long divisor;
 	long x;
 
+	divisor = limit - setpoint;
+	if (!(s32)divisor)
+		divisor = 1;	/* Avoid div-by-zero */
+
 	x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
-		    limit - setpoint + 1);
+		    (s32)divisor);
 	pos_ratio = x;
 	pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
 	pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
@@ -842,8 +847,12 @@ static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,
 	x_intercept = bdi_setpoint + span;
 
 	if (bdi_dirty < x_intercept - span / 4) {
+		unsigned long divisor = x_intercept - bdi_setpoint;
+		if (!(u32)divisor)
+			divisor = 1;	/* Avoid div-by-zero */
+
 		pos_ratio = div_u64(pos_ratio * (x_intercept - bdi_dirty),
-				    x_intercept - bdi_setpoint + 1);
+				    (u32)divisor);
 	} else
 		pos_ratio /= 4;